In an electrophotographic image forming apparatus of the visible-image transfer type, a beam of light carrying image information picked up from a given original document sheet is projected onto the photosensitive peripheral surface of a rotating image-transfer drum to produce electrostatic latent images corresponding to the image information. The latent images thus produced on the photosensitive peripheral surface of the image-transfer drum are developed into visible images on a suitable record medium with the agency of toner particles. An image forming apparatus of this type ordinarily has a control system for controlling the density to which images are to be reproduced or printed on a record medium The density of the images to be printed on a record medium can be regulated by controlling the quantity of the toner particles in the developing unit, the bias voltage to be applied to the developing sleeve of the unit, and/or the exposure volume with which the document sheet is to be optically scanned. Examples of such an image density control system are taught in Japanese Provisional Patent Publications (Kokai's) No. 58-23043 (Prior Art "I"), No. 60-133475 (Prior Art "II"), No. 60-146256 (Prior Art i "III"), No. 60-260072 (Prior Art "IV"), and No. 60-119580 (Prior Art "V"). Each of the systems of Prior Art "I" to "IV" depends for its operation on the detection of the density of the visible toner images produced on the photosensitive peripheral surface of the image-transfer drum to control some operational parameters used for the development of latent images into visible ones and the irradiation of an original document sheet. The parameters thus controlled through detection of the toner image density are corrected depending on the ambient temperature and humidity, the time duration for which the developer is to be agitated in the developing unit, and the possible fluctuations in the sensitivity of the drum. On the other hand, the control system of Prior Art "V" depends for its operation on the detection of the distribution of image densities on an original document sheet to determine a degree of contrast for the particular document for regulating the exposure volume so that such a degree of contrast is achieved for the image reproduced.
Problems are however encountered in each of these prior-art control systems. In a density control system of any of Prior Art "I" to Prior Art "IV" in which the density of the images to be reproduced on a record medium is controlled through detection of the visible image produced on the image-transfer drum, there is a problem in that the density of the images to be reproduced is determined without respect to the image density distribution and the degree of contrast of the original document sheet When a document sheet is to be reproduced which has fine linear patterns coarsely distributed within a white background area, the linear patterns are likely to be blurred or even blanked out in the images reproduced. If, conversely, a document sheet having fine linear patterns thickly distributed in a white background area is to be reproduced, the linear patterns tend to be bridged through fogs formed therebetween in the reproduced images. None of the prior-art systems of Prior Art "I" to Prior Art "IV" is furnished with means to cope with such problems
These problems are significantly alleviated in a control system of Prior Art "V" in which the degree of contrast for an image to be reproduced is controlled through detection of the distribution of image densities on an original document sheet The control system of this type uses a comparatively low voltage for the document exposure lamp and, for this reason, has a drawback in that there is a tendency to cause fogging on the entire area of the record medium. The fogging is caused especially in the presence of any foreign material or a stain in the optical system or in the event of any environmental change caused in the neighborhood of the photosensitive peripheral surface of the image-transfer drum.